Fuel jets are known for supplying fuel to the air flow passage of a carburetor or similar device where such fuel is mixed with air to form a combustible air/fuel mixture. Such jets are usually located in a venturi portion of the air flow passage, upstream of the throttle plate, such that the vacuum produced in the venturi portion helps draw fuel through the jet and into the air flow passage. Some carburetors may have multiple jets located in the air flow passage. For example, a slow jet may be provided for supplying fuel at idle or at low speeds and a main jet may be provided for supplying fuel at midrange or high speeds.
The main fuel jets used in such carburetors often have a flow controlling needle that is reciprocally contained within the fuel flow passage of the fuel jet. The amount of fuel delivered by the fuel jet is controlled by sliding the needle into or out of the fuel jet using an reciprocal slide that is mounted in the carburetor body. The upper end of the needle is connected to the slide such that vertical up and down movement of the slide is translated into vertical up and down movement of the needle. The needle includes a tapered section which creates a bigger gap between the outer diameter of the needle and the fuel flow passage of the fuel jet as the needle rises, thus increasing the fuel flow through the main jet as the needle rises in the main jet.
At idle or low speeds, all or most of the fuel supplied to the engine is supposed to be delivered by the slow jet. No or little fuel is supposed to flow through the main jet at this time. Thus, in the lowest position of the slide corresponding to the maximum insertion of the needle into the main jet, fuel flow should occur through the main jet only at a minimum value, i.e. zero flow or a preselected small flow.
However, the Applicant has discovered that some additional, unwanted fuel will flow up and out of the main jet even at idle or low speeds when fuel flow through the main jet should be at its minimum value. The Applicant has observed this happening with drops of fuel seeming to travel up the back of the needle in a wicking type manner. This fuel then enters the air flow in the air flow passage and overly enriches the air/fuel mixture at idle or at low speeds. This causes the engine to perform less well than it should at such engine operational conditions and can prevent a smooth acceleration from idle or low speed conditions. This is obviously undesirable.